This invention relates to a method and apparatus for exact measurement of pressures by means of level or pressure measuring equipment of the bubble tube type. Such equipment in principle comprises a bubble tube, a source of pressure, a pressure transducer, and valves controlled by a sequence switch. An important use of such equipment is for monitoring the water level in water-courses and reservoirs for hydro-electric power generation, in which it is of much great importance to obtain exact and reliable data regarding the reservoir content and variations therein.
In the above use, which is of particular interest in connection with this invention, strict requirements to such level measurement are set also for the reason that the measuring equipment or installations are as a rule located high in the mountains at places to which access is difficult and where periods of time may pass between times at which inspection and maintenance and possibly repairs can take place.
Even though the invention has resulted from the particular requirements of the above use and associated measuring problems, the solutions of the present invention are not restricted to this use, but can, of course, be used also in other fields in which corresponding requirements and needs with respect to measurement of pressures or level exist, for instance in large storage tank installations or in tankers.
Specific problems associated with measurements of this type partly relate to the economic utilization and simplification of the source of pressure, and in part to circumstances concerning the sensing element of the pressure transducer which in view of the requirements as to accuracy must have a very high sensitivity and therefore is subject to deterioration or damage upon adverse influence. Another consideration which is important when using bubble tubes for accurate measurements, is that the tubes employed in many instances will be subject to fouling and other depositions which over a period of the time can lead to a highly reduced flow cross-sectional area in the tubes.